The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andr e Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. EP-A-423399 and EP-A-410500.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR-element) or in the image-receiving layer of a so-called single-support element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in water permeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
According to a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB- 1,241,661 a support is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the emulsion layer. Printing plates of this type have a printing endurance typically around 10000 copies.
According to a second type a hydrophilic support, mostly anodized aluminum, is provided in the order given with a layer of physical development nuclei and a silver halide emulsion layer. After information-wise exposure and development the imaged element is treated to remove the emulsion layer so that a support carrying a silver image is left wich is used as a printing plate. Printing plates of this type have a higher printing endurance typically at least 25000 copies. Such type of lithographic printing plate is disclosed e.g. in U.S. Pat. No. 3,511,656, EP-A-278766, EP-A-410500 and EP-A-483415.
Said first type of mono-sheet DTR offset printing plates is not compatible with the second type of mono-sheet DTR offset printing plates with regard to dampening solutions and printing inks, which is cumbersome for the printer. In order that said first type of mono-sheet DTR offset printing plates shows no ink acceptance in the non-printing areas (no toning), use should be made of special printing inks and dampening solutions containing an amount of a transparent pigment, usually colloidal siliciumoxide as disclosed e.g. in U.S. Pat. Nos. 3,829,319, 4,238,279 and EP-A 304.662.
Dampening solutions containing an amount of a transparent pigment are however detrimental for use with the second type of mono-sheet DTR offset printing plates because of excessive chemical wear, causing a bad ink acceptance. Still further, such dampening solutions shows a lack of shelf life due to the presence of this transparent pigment in said solutions.